High-intensity arc lamps emit light with extremely high brightness for use in projection display systems, for example, conference room projectors, home theatre projectors, etc. Such lamps are powered by a direct current (DC) voltage ranging from 12 V to 25 V and a DC current ranging from 20 A to 50 A. Operating the lamp requires a high voltage ignition pulse of up to 35 kV, depending on the temperature and gas pressure within the arc tube of the lamp. An arc sustaining circuit supplies a sufficient current that sustains the arc for turning on the lamp. As a result, a special power supply, known as a ballast, is utilized for these lamps.
FIG. 1 shows a block diagram of a known high-intensity arc lamp ballast that powers a lamp 107 by an alternating current (AC) power source 101. The lamp ballast is composed of an EMI filter 102, a bridge rectifier 103, a power factor correction (PFC) circuit 104, a DC/DC voltage converter 106, an auxiliary power supply 108, an arc sustaining circuit 109, and an igniter 110. The PFC circuit 104 converts an AC input voltage 101 to a DC voltage, i.e., VB of 380 V˜400 V, and shapes the input current to reduce its harmonic contents and improve system efficiency. The full-bridge converter 106 converts DC voltage VB to a voltage required by lamp 107. The auxiliary power supply 108 generates suitable voltages for igniter 110 and arc sustaining of lamp 107.
More detailed description of the ballast circuit of prior art for high-wattage arc lamps can be made by referring to FIG. 2. The PFC stage is not shown in the figure and well known by those skilled in the art. Both full-bridge DC/DC converter 209 and auxiliary power supply 108 (e.g. a flyback converter) receives PFC output voltage VB as the input. The full-bridge DC/DC converter 209 is composed of switches Q3-Q6, DC voltage blocking capacitor Cb, transformer T4, diodes D1 and D2, and inductor Lig. Because lamp 107 has aging effect, i.e., the lamp impedance increases with time, full-bridge DC/DC converter 209 powers lamp 107 preferably with a constant-power control during normal operation to avoid excessive lamp power when a constant-current control is used. Flyback converter 108 converts VB to VC1 to provide an input for igniter 110 and an arc sustaining current through switch Q2 and current-limiting resistor R1 right after the lamp ignition.
When switch Q2 is turned on, the voltage at the cathode of diodes D1 and D2 becomes voltage VC1. The voltage at the anode of diodes D1 and D2 is the voltage across the secondary winding of transformer T4, which is equal to VB·(Ns/Np), where Np and Ns are the turn numbers of the primary and secondary windings of transformer T4, respectively. Voltage VC1 is typically in the range of 100 V˜200 V and ensures adequate arc sustaining current after lamp 107 is ignited. Assuming a VB of 400 V and an Ns/Np ratio of 3/28, the voltage at the anode of diodes D1 and D2 would be 43 V. This voltage ensures that diodes D1 and D2 do not conduct when switch Q2 is turned on since both diodes are reverse biased.
Igniter 110 of FIG. 2 has two stages. The first stage includes a resistor Rig1, energy storage capacitor Cig1, silicon diode for alternating current (SIDAC) 226, and transformer T1. SIDAC 226 conducts current in either direction but only after its breakdown voltage has been reached. Before lamp 107 is ignited, switch Q2 is turned on, and voltage VC1 provides a charging current which flows through switch Q2, resistor R1, and resistor Rig1 to charge capacitor Cig1. When the increased voltage across capacitor Cig1 turns on SIDAC 226, a voltage pulse is generated across the secondary winding of transformer T1, which charges storage capacitor Cig2. Capacitor Cig1 discharges quickly as SIDAC 226 conducts current. The voltage across capacitor Cig1 is charged up again when SIDAC 226 turns off as the current flowing through SIDAC 226 is lower than its holding current. This operation continues as long as switch Q2 remains on. The second stage of igniter 110 includes spark-gap 219, diode 227, and transformer T2. Once the voltage across capacitor Cig2 reaches the break-over voltage of spark-gap 219, a voltage pulse is generated across the secondary winding Lig of transformer T2 to strike lamp 107. The benefit of using a two-stage igniter is that the input voltage at the primary side of ignition transformer T2 is boosted by the first stage, thereby allowing the use of a lower turns ratio for the secondary-to-primary winding of transformer T2. A lower number of secondary turns decreases power loss at high current for lamp 107. The turning on or off of switch Q2 is controlled by a control circuit 229.
After lamp 107 is ignited, switch Q2 is kept on for a period of 100 μs-500 μs before it is turned off. During this period, energy-storage capacitor C1 is discharged, and a current flows through switch Q2, resistor R1, and winding Lig to sustain the arc in lamp 107. When the ignition period is over, igniter 110 stops generating voltage pulses as the maximum voltage across capacitor Cig1 becomes comparable with the operating voltage of lamp 107, which is well below the turn-on threshold of SIDAC 226. Meanwhile, spark-gap 219 is turned off, leading to an open-circuit condition for the primary side of transformer T2. Thus, the secondary winding of transformer T2 and its magnetic core form an inductor Lig. After switch Q2 is turned off, full-bridge DC/DC converter 209 takes over and provides the required DC current through inductor Lig for operating lamp 107.
As can be seen from FIG. 2, before lamp 107 is ignited, the voltage across diodes D1 and D2 is the sum of voltage VC1 and the reflected voltage VB·(Ns/Np) across the secondary winding of transformer T4. As a result, diodes D1 and D2 should have a voltage rating higher than the sum of VB·(Ns/Np) and VC1.
Assuming the voltage rating of diodes D1 and D2 is VD, VC1 needs to be lower than VD−VB·(Ns/Np) to ensure safe operation of these output diodes. Therefore, voltage VC1 for the igniter input is ultimately limited by the voltage rating of diodes D1 and D2. This leads to the choice of either larger size and less reliable igniters or output diodes with high voltage ratings but an accompanying higher power loss of the diodes and subsequent significant loss of efficiency.
Therefore, there exists a need for a power supply having low power loss and high efficiency for igniting and powering a lamp with an arc sustaining circuit.